[AMRadio] Interesting FM Bcast Propagation


doxemf at aol.com doxemf at aol.com
Sat Jul 15 21:06:52 EDT 2006


Hello,
I was listening to my usual station 93.9 North Hampton MA yesterday, ( 
Friday), which is about 50 miles or so east over the mountain
when it faded out and was replaced by 93.7 from Albany NY about 40 
miles west over another mountain.
This reception was swinging between the two stations in a similar 
fashion to AM QSB but without the phasing for several hours in the 
afternoon. Tuning in between the frequencies yielded the usual 
distortion and if I tuned to the 93.7 station it was receiving the same 
way with the cross fade between the two stations perfect.
Now the signals from both stations are normally just at or below the 
stereo detection point using an SAE 8000 analog tuner. But without the 
shifting. Antenna is a normal 300 ohm inside FM balanced dipole 
broadside E/W.
I did fire up a wide band receiver to look at the IF spectrum but the 
signals were so close to the noise floor that any measurements were not 
possible, although enough of the levels were visible to see the see-saw 
strength between the two stations.
There is a 1000 ft+ N/S mountain ridge immediate west and another one 
to the east about 5 miles.

This behavior has been heard several times especially when there are 
significant temperature gradients low in the atmosphere.
I would suspect that however the reflective or ducting effect is 
producing this could be visualized as similar to the waves
on the surface of a pond reflecting light at a slow rate. IE, 
consistent, slow ripples over a thermal reflective layer of the 
ionosphere.
I am just surprised at the short distance and would think that there 
would be technique to take measurements of the distance involved and 
determine the actual height of the reflective layer.

Just an interesting observation of propagation i had not observed 
before moving here to MA.

Has anyone here in the NE ever noticed similar propagation with 6 or 2 
mtrs?

Bill KB3DKS/1
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